1. Field of the Invention
This invention relates to a method and apparatus for simulating, indicating and controlling the temperature of electric power transformers of the type used by public utilities. These types of transformers typically have their transformer windings immersed in a liquid coolant. During operation of the power transformer it is important to know the temperature of the hottest transformer winding, the maximum temperature each winding has reached, when cooling fans and pumps should be turned on and off and when power to the transformer should be turned off.
2. Discussion of Prior Art
This invention represents a variation of that described in U.S. Pat. No. 4,745,571 which issued from application Ser. No. 772,133 filed Aug. 30, 1985 and Ser. No. 829,214 filed Feb. 14, 1986 of Joseph F. Foster. It is known from prior systems to simulate the winding temperatures by measuring the top oil temperature of the transformer and biasing the thermometer reading for each winding, by an amount proportional to the corresponding transformer load, or winding current, so as to simulate each winding temperature. Such a system employs thermometers with sensing elements measuring the top oil temperature, current transformers to provide signals proportional to transformer loads, heater coils to provide the bias and auto transformers to adjust the current supplied to the heater coils.
It is an object of this invention, rather than have biasing heaters, to modulate the actual oil temperature in a transformer, to provide a method and apparatus to electronically determine the winding temperatures based on the oil temperature in the transformer and the increased temperatures that result from currents flowing through the transformer windings.
It is a further object to provide an improved adjustable transformer winding temperature simulator, indicator and controller which may be adjusted or programmed to provide variable temperature increments to the oil temperature in accordance with the multi-phase transformer manufacturers specification for temperature change at various transformer loads or winding currents. A different temperature increment is provided for each relay output to accommodate accurate control, and a complete separate simulator circuit for each transformer winding.